The present invention relates to rotary magnetic position sensors used to measure angular displacements, and more particularly to magnet assemblies used therewith.
The use of magnetoresistors (MRs) and Hall devices as position sensors is well known in the art. For example, a magnetically biased differential MR sensor may be used to sense angular position of a rotating toothed wheel, as for example exemplified by U.S. Pat. No. 5,754,042.
Position sensors with digital outputs provide discrete position information only whereas an analog position sensor can provide both position information and outputs that can be used to drive an electric motor or other similar electromechanical devices. Many of these devices are driven by sinusoidal excitations as a function of position. Consequently, an analog position sensor having an output that varies sinusoidally with position could be used to generate absolute angular positions as, for example, an electrical power steering system to measure the angle of rotation of the steering wheel, and/or reference signals to produce the desired sinusoidal phase drive currents and voltages to drive electric motors and other similar electromechanical devices. Applications may further include throttle position sensors, pedal position sensors in drive-by-wire systems, body height position sensors for automotive suspension systems, a 3-phase sine generator for brushless motors, a sine/cosine resolver for servo motors, etc.
The operational principle of an angle encoder sensor 10, shown at FIGS. 1 and 2, is based upon the property of Hall plates and semiconductor magnetoresistors, collectively referred to herein as magnetosensitive devices 12, to sense only the normal component of the magnetic field B which passes through them as relative rotation R occurs about a rotation axis A, wherein the field is provided by two permanent magnets 14, 16 that are mutually spaced apart a distance g, wiring 18 provides a current connection to an external circuit. Consequently, if a constant and uniform magnetic field is rotated in the plane perpendicular to the surface of a magnetosensitive device, the output signal will vary as the cosine of the angle between the direction of the incident magnetic field lines and the line normal to the surface of the device, that is, in proportion to Bxc2x7Cos xcex1, wherein xcex1 denoted the angular rotation and the angle of the flux lines relative to the normal of the surface of the sensor. It is preferred in this regard, that the magnetosensitive device be linear in its response to change in direction of the incident magnetic field, such as that provided by Hall plates; however, magnetoresistors operating in their linear region can also be used. In addition, operation over any ambient temperature range may require temperature compensated magnetosensitive devices. Also, it should be noted that included by the term xe2x80x9cmagnetosensitive devicesxe2x80x9d are ferromagnetic magnetoresistors, including giant magnetoresistor (GMR) sensors, which can also be used although these are less preferred because their resistance versus magnetic flux density saturates at a relatively low level compared with Hall sensors that do not saturate.
While a conventional magnet assembly having two structurally independent magnets functions admirably, it has drawbacks. For example, there is a duplication cost associated with providing two separate magnets, and there is an associated assembly operation with its own inherent problems, including for example the possibility of mislocation or misalignment of the magnets relative to each other, relative to the magnetosensitive device and/or relative to the rotation axis, thus necessitating precautions which add further to the cost of assembly.
Accordingly, what remains needed is a compact, inexpensive contactless position sensor having a sinusoidally varying output suitable for sensing schemes, wherein the magnet assembly thereof is simply constructed.
The present invention is an analog angle encoder having a simply constructed magnet assembly, wherein rotation of a magnetic field relative to a magnetosensitive device provides a varying output of the magnetosensitive device that varies in a predetermined manner (preferably sinusoidally) with respect to the angle of relative rotation.
The analog angle encoder according to the present invention includes a simply constructed magnet assembly and a magnetosensitive device situated in the magnetic field provided by the simply constructed magnet assembly such that relative rotation between the magnetic field and the magnetosensitive device provides a predetermined signal indicative of the angular position of the magnetosentivie device relative to the direction of magnetic field incident thereto.
Further according to the present invention, the simply constructed magnet assembly is composed of a single piece of magnetic material wherein only a first portion thereof is magnetized, and this first portion provides the aforesaid magnetic field for the sensor. The remaining (second) portion of the magnet material remains unmagnetized. This second (unmagnetized) portion does not substantially influence the magnetic field produced by the first (magnetized) portionxe2x80x94any more than, for example, air would cause.
Generally, the simply constructed magnet assembly may have any suitable closed-path geometry, wherein provided are two symmetrically disposed, mutually attracting magnetic poles which provide a suitably uniform magnetic field in the air gap therebetween. Advantageously, because the magnet assembly is simply constructed, meaning that it is of a single piece construction, the placement of the magnet poles is precise and cannot drift over time.
According to a preferred mode of making, the magnetic material is of ferrite, preferably of the ceramic-cobalt type, which may be molded (as for example by extrusion) into a desired shape, machined into the desired shape, or some combination thereof. Magnetization of the first portion of the magnet assembly may, for example, be performed using a magnetizer with pole pieces shaped to induce magnetization only at the first portion. The second portion, unmagnetized by the magnetizer, will behave magnetically (have a permeability) similar to air.
Accordingly, it is an object of the present invention to provide an analog angle encoder having a simply constructed magnet assembly, wherein a signal is produced responsive to sensed relative rotation between a magnetosensitive device and the applied magnetic field is provided by the simply constructed magnet assembly.
This, and additional objects, features and benefits of the present invention will become apparent from the following specification.